1. Field of the Invention
The invention relates to portable gyroscopic compass devices of the kind adapted for surveying purposes and more particularly relates to portable gyrocompass surveying devices including means for rapidly aligning the reference device with respect to geographic north.
2. Description of the Prior Art
The accuracy of any gyrocompass in seeking north is primarily determined by how well torques about its vertical axis are minimized. In practice, this has resulted in the past in concepts that attempt to minimize friction in the vertical axis support system by carrying some or all of the weight of the sensitive element along this axis on a small suspension wire or tape. As a consequence, a follow-up servo is required to minimize twisting in the supporting wire, since any twist produces undesirable elastic restraint torques. These techniques have proven to be successful in producing accurate gyrocompasses for marine navigation purposes; however, their implementation involves considerable complexity. Furthermore, they do not eliminate the error torques produced either by slip rings or by wire harnesses required to transmit electrical power to the sensitive element and therefore lack full suitability for use in surveying compasses.
The typical pendulous navigation gyrocompass requires a considerable time to settle on a meridian; one hour is not unusual. To decrease in significant degree the settling time only by varying the gyro-pendulum parameters, the concept selected must either reduce angular momentum of the instrument or increase the pendulosity of the sensitive element. Neither of these alternatives is fully satisfactory. Either one results in increased error from external vibration; decreasing the angular momentum results in a less accurate unit for a given torque about the vertical axis. Other techniques devised for fast settling also suffer from these and other fundamental disadvantages.
In the use of gyrocompass apparatus for land and other surveying purposes, it is often imperative that a portable gyrocompass reference be set up easily and that it provide useful data in a very short time period. While the one hour time that might be required to put a navigation gyrocompass into reliable indicating condition on a commercial marine vessel may be tolerated since the compass, once started, normally remains in continuous operation during the voyage, such extreme times are never acceptable in surveying instruments.
A recent attempt to overcome the aforementioned problems is reflected in the E. S. Rocks et al U.S. Pat. No. 3,419,967, issued Jan. 7, 1969 and assigned to the Sperry Rand Corporation. In this concept, considerable improvement in freedom and accuracy of operation was achieved by floating the sensitive instrument in neutral buoyancy in a binnacle with the sensitive element only partly immersed in the floatation fluid. The problem of undesired restraining torques during operation of the apparatus was further reduced by placing the rotor-driving battery within the sensitive float itself. This instrument was found to be relatively inexpensive and to be acceptable in certain surveying applications where a moderate settling rate was acceptable, but to have certain limitations now overcome by the present invention. However, when the Rocks et al device is designed to have pendulosity low enough to demonstrate high immunity to vibration, the time required to settle on north was found to be as long as ten times the settling time of the present invention. Additionally, it was experimentally observed that when fluid damping was adjusted for permitting a good settling time, a high sensitivity to tripod motion was demonstrated. Placing the battery in the floated element makes servicing difficult and yields a poor momentum-to-weight ratio. Furthermore, reading out of the azimuth angle requires the operator to auto-collimate with respect to a mirror surface on the floated assembly.